In construction sites, collisions between machines and workers present a significant occupational hazard. Operators of machines such as trucks, forklifts, cranes and the like may have difficulty seeing these workers, especially when the vehicle is reversing or maneuvering, because of the size and configuration of these machines or their cargo loads. Meanwhile, the workers may not notice when the vehicles are set in motion if the workers are preoccupied with their tasks when working near the machine.
Several approaches have been developed to reduce this risk. One conventional approach is to equip the vehicle with a beeper system in which a beeper automatically activates when the reverse gear of the vehicle's transmission gear is selected. However, the sound of the beeper may be drowned out by ambient noise or attenuated by hearing protection equipment worn by workers. Further, the sound of the beeper may become so prevalent in a busy construction site that workers may ignore the sound. Finally, the system ultimately relies on the worker's ability to move out of vehicle's path, which may not be possible if the worker is confined between the vehicle and another obstacle.
U.S. Pat. No. 7,388,479 discloses a back-up alarm system comprising an optical sensor, a circuit control and a speaker. The optical sensor is used to detect the presence of a worker when the vehicle is backing up. If the optical sensor does not detect the presence of a worker, the circuit control causes the speaker to generate a lower volume audible alarm. Conversely, if the optical sensor detects the presence of a worker, the circuit control causes the speaker to generate a higher volume audible alarm. Although this system is more selective as to when it produces the audible alarm than the conventional beeper system, it continues to suffer from many of the same disadvantages as the conventional beeper system.
U.S. Pat. No. 8,115,650 and US patent application Ser. No. 2011/0227748 disclose warning systems comprising a transmitter and a sensor onboard the vehicle, and passive RFID tags embedded in clothing worn by a worker. In general, the transmitter generates an interrogation signal within a detection zone. Any passive RFID tags within the detection zone emit a response signal. If the sensor detects the response signal, a processor actuates a sensory alarm or a means for stopping the vehicle. U.S. Pat. No. 8,115,650 also discloses that the system may include an ultrasound sensor onboard the vehicle that is activated in response to the detection of an RFID tag by the sensor to determine the distance between the vehicle and the worker. Depending on whether the worker is moving towards or away from the vehicle, the system selectively either allows the vehicle to continue moving or causes the vehicle to shut down. One potential disadvantage of using passive RFID tags is that they radiate relatively weak signals. This practically limits the size of the detection zone to a relatively short distance of about five meters or less, which is insufficient for rapidly moving vehicles. The lack of signal strength also makes it difficult to accurately determine the distance between the vehicle and the worker from the signal strength. Another disadvantage of systems using passive RFID tags is the need for reliable two-way signal transmission and reception between the vehicle and the worker. The system will fail if either one of the interrogation signal or the response signal is not both properly transmitted and received.
Accordingly, there is a need in the art for a method and system for reducing the risk of a machine colliding with an object, and in particular a person. The method and system should preferably be simple, economical, reliable and adaptable to warn vehicle operators of the presence of workers at significant distances from the machine.